Heroku removed the need for it with Rails 5

https://devcenter.heroku.com/articles/getting-started-with-rails5#heroku-gems

Closes railsbridge#569

Author: wassimk

sites/en/installfest/deploy_a_rails_app.step

@@ -73,7 +73,6 @@ step "Deploy your app to Heroku" do
 
       group :production do
         gem 'pg'
-        gem 'rails_12factor'
       end
     RUBY
 

sites/en/intro-to-rails/_deploying_to_heroku.step

@@ -23,7 +23,6 @@ end
 
 group :production do
   gem 'pg'
-  gem 'rails_12factor'
 end
   RUBY
 end